This invention relates generally to a process for forming antireflection coatings, and more specifically to a process for forming integral, multiple layer antireflection coatings and devices using these coatings.
The amount of reflection from a surface is a function of the index of refraction of the material on either side of that surface. For example, for an optical lens in air, the amount of light reflected from the surface of the lens is a function of the index of refraction of the material making up the lens and the index of refraction of air. It is well known in optics that the reflection from a surface can be reduced by providing that surface with an antireflection layer. An antireflection coating for the lens in air is a material having an index of refraction intermediate that of the lens material and air.
The effectiveness of a antireflection coating is a function of the indices of refraction involved, the thickness of the antireflection layer, and the wavelengths of light involved. A conventional antireflection coating, for example, consists of a dielectric layer that has an optical thickness equal to one quarter of the wavelength of the incident light (as measured in the dielectric) at which the minimum reflectance is desired. Reflectance as a function of wavelength increases sharply on either side of this minimum. For applications in which reduced reflectance is desired over a wide range of wavelengths (such as minimizing reflection losses over the usable solar spectrum for solar cells), a multiple layer antireflection coating can be used providing that each successive layer of the coating has an index of refraction less than the layer immediately below and greater than the layers above. The use of such multiple layer antireflection coatings is described, for example, in the text Physics of Thin Films, Advances in Research and Development, Academic Press, 1964, edited by Hass and Thun, pages 239-304.
Multiple layer antireflection coatings have been achieved, for example, by multiple depositions of dissimilar materials, each with its own index of refraction. Such multiple depositions are time-consuming, expensive, and often difficult to achieve. Careful consideration must be given to chemical compatibility, thermal expansion coefficients, and stresses resulting from the deposition processes used and inherently from the different materials used.
On view of applications requiring multiple layer antireflection coatings and of the difficulty in providing such coatings by prior art methods, a need existed for an improved process for forming multiple layer antireflection coatings which would overcome difficulties attendant with the prior art processes.
It is therefore an object of this invention to provide an improved process for providing an integral, multiple layer antireflection coating.
It is a further object of this invention to provide an improved process for reducing the reflectance of a semiconductor substrate.
It is a still further object of this invention to provide a process for fabricating an improved photosensitive semiconductor device.
It is yet another object of this invention to provide a process for an improved photovoltaic cell.